This invention relates generally to an apparatus for automatically strapping bales of cotton or other fibers or stacks of lumber or bricks or other items that are suitable for strapping. This invention relates more particularly to a system and method for welding the ends of thermoplastic straps together so as to form bales of cotton or so as to bind together any other desired material or items.
In the cotton or fiber industry, the normal method of banding or tying cotton bales has been to have workmen direct a tie, such as a band or wire, around a pressed cotton bale and then secure the ends of the ties appropriately, depending on the design of the tie. In the cotton or fiber industry, there are generally three ways to secure a bale after the bale has been pressed. Pertinent securing means include pre-formed steel wires that have interlocking ends pre-formed to define loops which engage one another during a tying operation, flat ribbon-steel bands which have their ends inserted into a crimp by which they are secured, and flat thermoplastic strapping material, typically formed of polypropylene or polyester, which has its ends welded together.
The steel pre-formed wires have a loop manufactured into each end thereof. The ends are interlocked around to form a square knot. When the pressure is released from the bale, the knot formed by the interlocking loops pulls tight and retains the bale against further expansion. In a conventional bale-tying operation, two workmen (one on each side of the baling press) manually bend the wires around the bale and secure the ends of the wires together in a wire tie guide assembly. The wires are normally tied together sequentially, one at a time.
Alternatively, the wires might be tied in a hydraulically operated wire tying device mounted on a baling press. The hydraulically operated wire tying device ties a plurality of wires having pre-formed interlocking ends around a bale formed in the press. Pivotally mounted wire bending assemblies take the place of workmen on each side of the baling press. The pivotally mounted wire bending assemblies bend the tie wires around the bale by inserting the ends of the tie wires into a wire tie guide assembly. However, workmen are still required to individually load each of a plurality of tie wires into the wire bend assemblies.
Although an improvement over the manual bale tying operation, the hydraulically operated wire tying device still exhibits certain problems which slow the baling process. Exact timing is required for the sequence of events which make up the wire tying operation. If a wire does not follow the correct path at the correct time, several factors can combine to prevent the interlocking ends of the wire from engaging to form a knot.
In particular, the interlocking ends of the wires are conventionally oriented such that the loops are disposed in a generally horizontal plane. This geometric orientation forces the wire closers to be constructed with relatively wide cavities, in order to accommodate the wide aspect ratios of the loops. This, in turn, allows the wires a greater degree of freedom of movement within the cavities. Consequently, there is a greater probability of one wire merely sliding past another, without their loops engaging in a knot.
In addition, press wear, both alone or in combination with component manufacturing tolerances, can cause a follow block to vary its position or orientation both vertically or from side to side. Consequently, the wire bend assemblies may not be in alignment with the wire tie guide assemblies. All of the above-described cases result in mis-ties, with a consequent undesirable loss of time and possible damage to the press.
Bale tying using flat steel straps is hindered primarily by. the cost of the strapping material, the complexity of the machinery used, and the speed at which the machinery is able to operate. In addition, both the weight of steel strap tie material and its substantially sharp edges make it cumbersome and particularly dangerous to handle.
Further, once it is removed from a bale, steel strapping material is not easily recycled by an end user. Removal is difficult, and once removed, a large volume of sharp material must be colleted and crushed together to form the material into a package that can be more easily handled.
Additionally, steel strap tie material is further disadvantageous in that its weakest point (the joint) is located in the highest stress position on the bale. This is true because the forming machinery is only able to apply a joint, i.e., a crimp, on the side of the bale (the position of the bale with the highest degree of lateral pressure or stress). This non-optional position of the crimp results in significant tie breakage with a consequent loss of bale integrity.
Conversely, plastic or non-ferrous strapping is an ideal material for strapping bales of cotton or other fibers. Indeed, such plastic strapping may be used to strap a wide variety of different items, such as lumber or bricks, as well as many other materials which are suitable for such strapping. As those skilled in the art will appreciate, plastic is relatively light in weight and can be formed into a variety of widths and thicknesses. Plastic also has comparatively soft or non-sharp edges which allows for easy handling. Its reduced weight lowers shipping costs. This plastic or non-ferrous strapping material is very competitive with both wire ties and metal strapping, on a cost per bale basis. Further, plastic strapping is easily adaptable to fully automatic welding machinery. Plastic strapping material is readily recyclable by the end user and is considered substantially safer than steel strapping material, particularly in instances of strap breakage wherein the sharp edges of the steel strapping frequently move violently and dangerously in response to breakage.
Because of the particular orientation of conventional plastic strap automatic tying machinery, certain disadvantages arise when one adapts strapping and joint forming apparatus to the structure of a baling press. Typically, automated thermoplastic strapping machinery, including a material feeder, tensioner, cutting shear and joint former, are so large that they are precluded from being able to be placed anywhere except on the side of the bale. As was the case with steel strapping material discussed above, thermoplastic strapping joint formation therefore takes place in the region of the bale that exhibits the highest degree of tensile stress.
In this regard, conventional thermoplastic strapping machinery must typically wait until a baling press has completed operation and has reached xe2x80x9cshut heightxe2x80x9d before it begins the strapping operation. The strapping head pulls strapping material off of a spool and directs it around the bale through a series of shoots, until the front edge of the strapping material has completed its circuit of the bale and is directed back to the region of the strapping head. The strap is then pulled tight around the bale to a pre-determined tension and the strap is then cut with a shear. The two ends are then joined by a friction weld, hot knife weld, or other similar joint forming operation, and maintained together until the joint is cool, in which time the strap is released and allowed to carry the tensile load of the bale.
Referring now to FIGS. 1a, 1b and 1c, there is shown a semi-schematic view of cotton, or other fibers, being pressed into a bale 14 between the platens 24 and 25 of a hydraulic press 16, 23 in accord with the prior art. Typically, fiber is pressed by a large hydraulic cylinder out of a box that measures approximately 30 inches wide by 54 inches long and 144 inches deep. Such a box is typically filled with approximately 500 pounds of cotton lint which is subsequently pressed into a 20 inch by 54 inch bale 14 measuring approximately 20 to 22 inches tall (in accordance with the illustration of FIG. 1a). The box from which the bale 14 is pressed has been omitted for the sake of illustrational clarity.
Strapping material, in the form of thermoplastic straps 26, is inserted through guide slots in the upper 24 and lower 25 platens, and are secured on the sides of the bale 14 (as shown in the illustration of FIG. 1b ). Once the bale 14 is tied, the press 16, 23 is released and the bale 14 is free to expand to the constraints of the straps 26. As shown in the illustrated embodiment of FIG. 1c, the bale 14 is then dumped out of the press 16, 23, making way for a subsequent box loaded with an additional 500 pounds of cotton lint for pressing into the next bale.
It should be noted that conventional thermoplastic strapping systems typically consist of three laterally spaced-apart strapping heads, such that the unit must be indexed in order to tie the requisite number of straps (typically 6) about a bale 14. Should the baling press leak down slightly (a typical artifact of cotton presses) the compressed bale would tend to grow as the press platens separated. When an indexing strapper is used, typically the #1, #3 and #5 straps are tied first. Five to ten seconds later, the strapping head is indexed and the #2, #4 and #6 straps are tied. In the event of press leakage, the first three straps are pulled tight around a smaller diameter bale. The second three straps are subsequently pulled tight around a bale that has expanded and are therefore not as tight. This causes the first three straps to be subject to substantially greater pressure than the second set. These ties are more prone to exceed yield strength and fail which typically causes total strap failure as pressure promptly increases for the ties of the second set.
Accordingly, an apparatus (and process) for tying bales with a flexible thermoplastic strapping material, that is designed for efficient, repeatable operation with low joint stress is needed. Such an apparatus should be designed for easy operation by a single workman to reduce labor costs, while at the same time being easy to install or retrofit to existing presses. Such an apparatus should further be mountable to operate in conjunction with a press such that ginning speed is increased by incorporating the tying process into the last few seconds of the bale pressing operation, thus eliminating the separate indexing and tying steps conventionally undertaken at the end of the process.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a method and apparatus for welding baling straps and the like. Two strap portions are received intermediate two weld plates such that the two strap portions overlap one another at least partially. The weld plates are moved back and forth with respect to one another, so as to similarly move the strap portions back and forth with respect to one another. As those skilled in the art will appreciate, such movement of the strap portions back and forth with respect to one another generates a substantial amount of heat. The resulting friction softens the interfacing region of the two strap portions so as to facilitate welding thereof.
According to one aspect of the present invention, the weld plates are rocked while the weld plates are moved back and forth. Rocking the weld plates while moving the weld plates back and forth effects the formation of a weld which attaches the two strap portions together in a manner having enhanced tensile strength.
According to another aspect of the present invention at least one, preferably both, of the weld plates comprises a base having a plurality of teeth formed thereon. At least a portion of the teeth formed upon the base are configured so as to grip a strap. The teeth are formed so as to define a longitudinally alternating pattern according to which portions of the weld plate vary in the ability thereof to grip the strap. Thus, some portions along the length of a weld plate grip the strap securely, while other portions along the length of the weld plate facilitate at least some longitudinal movement of the strap with respect to the weld plate. By providing alternating portions of the weld plate which alternately grip and allow movement of the strap, additional movement of the strap is facilitated which results in enhanced tensile strength of the weld.